A heretofore known capacitor-discharge ignition system apparatus is, for example, disclosed on page 83 of the book "Modern Electronic Circuits Reference Manual", McGraw-Hill Book Company, 1980. This known ignition system uses a switchable DC-DC converter formed of NAND-gates and running at a frequency of 10 kHz. Placed in series with a capacitor discharging thyristor is the primary winding of a transformer whose secondary winding triggers another thyristor to generate a pulse for blocking operation of the DC-DC converter. The duration of this blocking-pulse must be sufficiently long to delay restart of the converter while the main thyristor remains conductive, i.e. ionized. If the converter is restarted too soon, the thyristor will immediately conduct current again without being triggered at its gate-electrode by an ignition pulse. In addition, the selected blocking period for the converter must be sufficiently long to avoid a restart of the firing-capacitor recharging converter before the thyristor is completely de-ionized, i.e. in its non-conductive state, having discharged a maximum possible voltage joined with a maximum possible current through the thyristor.
In this prior art arrangement, the maximum ignition frequency is limited so as to generate sufficient high voltage for the firing-capacitor. Indeed, the period of blocking the converter of this known apparatus is varied in relation to the ignition frequency, further reducing the blocking period of the converter with increasing frequency. This is accomplished by placement of the transformer in the current path of the main thyristor which, after a pulse has been triggered, triggers the second thyristor to provide a blocking pulse, delayed by an R-C combination, to the NAND-gates. Although the blocking period decreases with increasing frequency, its proper length can only be approximated with respect to the actual condition of the thyristor; the blocking period must always be larger, and include a safety margin, than the physical absolute possible minimum period. As the transformer can transmit only current changes, a falsely triggered thyristor cannot be reverted from its permanent conductive condition unless and until the holding current is interrupted by switching off of the power source.
Moreover, the discharge time of the firing capacitor is primarily influenced by the impedance of the ignition coil. In the published prior art schematic, unless the blocking period is selected with regard to the largest possible impedance of an eventually-employed ignition coil, the apparatus must be optimized to the particular ignition coil to be used and production tolerances of the coil must be taken into consideration. If in addition the spark power is altered through variations in apparatus component values, the blocking-period must again be timed by R-C adjustment.